Method of surface treatment of zinc-containing metal substrate, and surface-treated zinc-containing metal substrate

ABSTRACT

A brass-plated steel cord 1A is immersed in an organic solvent 13. The steel cord that was immersed in the organic solvent 13 is immersed in an aqueous alkaline solution 22. Next, the steel cord 1A that was immersed in the aqueous alkaline solution 22 is immersed in an aqueous silane coupling solution 32. Finally, the steel cord 1A that was immersed in the aqueous silane coupling solution 32 is heated. Thus is manufactured a steel cord 1B equipped with the silane coupling agent, wherein the silane coupling agent is excellently bonded to the surface.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation Application of PCT International Application No. PCT/JP2019/020206 filed on May 22, 2019, which claims priority to Japanese Patent Application No. 2018-101441 filed on May 28, 2018, the entire disclosures of the applications being considered part of the disclosure of this application and hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to a method of surface treatment of a zinc-containing metal substrate, as well as a surface-treated zinc-containing metal substrate surface-treated by this surface treatment method. The zinc-containing metal substrate includes both a substrate which itself is a zinc-containing metal, as well as a substrate which does not contain zinc but whose surface is plated with a zinc-containing metal.

BACKGROUND OF THE INVENTION

Many automobiles, automatic doors, wire saws, assembling machines, automated machines, packaging machines, printing machines and the like employ power transmission belts made of an elastomer. In order to improve durability or the like, often a slender reinforcing material is embedded within a belt, which is made of elastomer, along the longitudinal direction thereof. A steel cord of excellent tensile strength is adopted as one reinforcing material.

A typical material used for a power transmission belt is rubber. In recent years, however, urethane resin, which excels in durability, wear-resistance and the like, has also come to be adopted.

In order to improve the adhesion strength between a urethane resin and a steel cord embedded within the urethane resin, it is known to form a film of a silane coupling agent on the surface of the steel cord. A silane coupling agent has a reactive group that chemically bonds with an inorganic material and a reactive group that chemically bonds with an organic material, and strongly bonds an inorganic material (such as a steel cord) to an organic material (such as urethane resin), which usually are very difficult to join.

In chemical bonding between an inorganic material, say a steel cord, and a silane coupling agent, use is made of SiO bonding (silicon-oxygen bonding) (siloxane crosslinking) formed by causing the silane coupling agent to adhere to the steel cord and causing condensation after hydrolysis. In order to enhance SiO bonding, JP Patent No. 5588246 describes the cleaning of the steel cord with an acid-containing liquid having a pH of less than 7, and JP Patent Nos. 5588247 and 6214407 describe the cleaning of the surface of a steel cord with an alkaline liquid (having a pH that exceeds 7).

Water-soluble oil adhering to the surface of a steel cord can be removed by washing the steel cord using an acidic or alkaline liquid. However, there are instances where a steel cord is coated not only with oil that is water-soluble but also with oil that is non-water-soluble, and in such cases the non-water-soluble oil cannot be removed sufficiently by an acidic or alkaline liquid.

Further, in order to so arrange it that water-soluble oil will not be allowed to remain on a steel cord as much as possible, it will suffice if the steel cord is immersed continuously in an alkaline liquid over a long period of time. However, the longer the immersion time, the lower the yield of product manufacture. In addition, when a brass-plated steel cord is immersed continuously in an alkaline liquid for a long period of time (ten minutes or more, for example), the brass becomes discolored and surface gloss is lost.

BRIEF DESCRIPTION OF THE INVENTION

An object of the present invention is to provide a substrate surface-treatment method for achieving strong adhesion to an organic material by using a silane coupling agent.

A further object of the present invention is to provide a surface-treated substrate capable of being strongly adhered to an organic material.

A method of surface-treating a zinc-containing metal substrate is characterized by: bringing an organic solvent into contact with the surface of a substrate having a zinc-containing metal on a surface layer thereof; bringing an aqueous alkaline solution, for example an aqueous sodium hydroxide, into contact with the surface of the substrate that was brought into contact with the organic solvent; bringing an aqueous silane coupling agent solution into contact with the surface of the substrate that was brought into contact with the aqueous alkaline solution; and heating the substrate brought into contact with the aqueous silane coupling agent solution. The substrate having the zinc-containing metal on its surface layer may be one where the substrate itself is composed of a zinc-containing metal (that is, the substrate is one made of a zinc-containing metal), or one where the surface layer of a material (steel, for example) not containing zinc is plated with a zinc-containing metal. In either case, the zinc-containing metal substrate surface-treated by the present invention is equipped with a zinc-containing metal on the surface that comes into contact with the outside air. The zinc-containing metal includes zinc itself as well as an alloy (brass, for example) that includes zinc.

The silane coupling agent is used in a liquid-phase state in which it has been dissolved in water. From the standpoint of water solubility, an amino-based silane coupling agent can be used suitably as the silane coupling agent.

In the step of processing the zinc-containing metal substrate (a manufacturing or molding step), there are instances where the zinc-containing metal substrate is coated with water-soluble oil and non-water-soluble oil used as lubricant. According to the present invention, the surface of the zinc-containing metal substrate is cleaned with an organic solvent and is cleaned also with an aqueous alkaline solution. In particular, since the non-water-soluble oil, which is difficult to remove with the aqueous alkaline solution, can be eliminated with the organic solvent, oils clinging to the surface of the zinc-containing metal substrate can be removed sufficiently regardless of their water-solubility and non-water-solubility.

Due to the fact that the zinc-containing metal substrate surface-processed in the present invention has the zinc-containing metal on its surface layer, when the substrate is brought into contact with an aqueous alkaline solution, not only is the water-soluble oil removed, but, in addition, the zinc and a base on the surface of the zinc-containing metal substrate chemically react so that a hydroxyl group is introduced to the surface of the zinc-containing metal substrate. Since the surface of the zinc-containing metal substrate is cleaned sufficiently by undergoing the cleaning process using the organic solvent and aqueous alkaline solution, many hydroxyl groups can be introduced to the surface of the zinc-containing metal substrate. The greater the number of hydroxyl groups on the surface of the zinc-containing metal substrate (the smaller the amount of oil that remains on the substrate surface), the greater the number of hydrogen bonds between silanol groups and hydroxyl groups formed on the surface of the zinc-containing metal substrate owing to contact with the aqueous silane coupling agent solution, and there is also an increase in the number of SiO bonds formed by a dehydration condensation reaction that is based on the subsequent heat treatment. Specifically, according to the present invention, there is provided a zinc-containing metal substrate equipped with such a surface property that the surface easily reacts with the silane coupling agent, wherein the silane coupling agent chemically bonds readily with the surface.

Owing to the fact that the surface of the zinc-containing metal substrate has many SiO bonds, the surface-treated zinc-containing metal substrate that has undergone the surface treatment according to the present invention easily reacts with an organic material and the zinc-containing metal substrate and organic material can strongly adhere (chemically bond) to each other.

In an embodiment, an organic solvent is brought into contact with the surface of the zinc-containing metal substrate by immersing the zinc-containing metal substrate in the organic solvent. Preferably, an organic solvent tank filled with the organic solvent is placed inside a water tank filled with water, the water tank is subjected to ultrasonic vibration and the zinc-containing metal substrate is immersed in the organic solvent vibrated ultrasonically via the water. The zinc-containing metal substrate in one embodiment is a filament and, by placing the organic solvent tank in the travel path of the filament, the organic solvent can be made to continuously contact the zinc-containing metal substrate, which is the filament. The zinc-containing metal substrate may be obtained by twisting together a plurality of filaments. For example, the zinc-containing metal substrate may be a twisted wire (a brass-plated steel cord) obtained by twisting together multiple steel wires each of which has been brass-plated. By applying ultrasonic vibration to the organic solvent, the twisted wire, inclusive of the interior thereof, can be cleaned and the strength of adhesion to the organic material can be improved. Further, the danger that the organic solvent will ignite can be prevented by causing the ultrasonic waves to propagate toward the organic solvent via the water.

With regard to the aqueous alkaline solution as well, the aqueous alkaline solution may be brought into contact with the surface of the zinc-containing metal substrate by immersing the zinc-containing metal substrate in an aqueous alkaline solution tank filled with the aqueous alkaline solution, and the aqueous alkaline solution may be vibrated ultrasonically.

As described above, by cleaning the surface of the zinc-containing metal substrate with an organic solvent and even with an aqueous alkaline solution, oil on the surface is removed sufficiently and many SiO bonds are formed (a film is formed) on the surface. However, even supposing some oil remains on the surface, it is possible to cause strong adhesion to the organic material (to manifest a pull-out load equal to or greater than a predetermined value). The longer the periods of immersion in the organic solvent and in the aqueous alkaline solution, the greater the amount of oil that is removed from the surface of the zinc-containing metal substrate. However, the longer the immersion times, the lower the production yield and the more discoloration occurs in the zinc-containing metal substrate. That is, there is little merit in completely removing oil from the surface of the zinc-containing metal substrate (in elevating the degree of oil removal more than necessary).

The present invention provides a surface-treated zinc-containing metal substrate which is premised on the fact that oil remains on the surface of the zinc-containing metal substrate but the substrate can be made to strongly adhere to an organic material. The surface-treated zinc-containing metal substrate according to the present invention is characterized in that, when an amino compound and oil are adhering to the surface of a substrate that has a zinc-containing metal on a surface layer thereof, and a is peak intensity of the amino compound and b is peak intensity of the oil, a peak intensity ratio A represented by A=a/b is equal to or greater than 2.6. In the surface treatment method described above, the amino compound adheres to the substrate surface owing to use of an aqueous solution of an amino-based silane coupling agent. The oil adheres to the substrate surface when the substrate is processed (when it is manufactured or molded). The peak intensity a of the amino compound and peak intensity b of the oil on the substrate surface can be measured by, for example, pyrolysis gas chromatography.

With the peak intensity ratio A, which is obtained by dividing the peak intensity a of the amino compound by the peak intensity b of the oil, serving as a parameter, it was confirmed that, when the adhesion strength (pull-out load) between the surface-treated zinc-containing metal substrate and the organic material was measured while variously changing the peak intensity ratio A, a function holds in which the peak intensity ratio A is a variable. At a peak intensity ratio A equal to or greater then 2.6, a surface-treated zinc-containing metal substrate to which an amino compound and oil adhere is such that the strength of adhesion to the organic material (the pull-out load) is greatly improved in comparison with a zinc-containing metal substrate subjected to no surface treatment whatsoever.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a process for surface treatment of a brass-plated steel cord;

FIG. 2 schematically illustrates, by means of a structural formula, chemical bonding that occurs on the surface of a steel cord that has undergone a hydroxide treatment;

FIG. 3 schematically illustrates, by means of a structural formula, chemical bonding that occurs on the surface of a steel cord that has undergone treatment with a silane coupling agent;

FIG. 4 schematically illustrates, by means of a structural formula, chemical bonding that occurs on the surface of a steel cord after a dehydration condensation reaction;

FIG. 5 is an enlarged cross-sectional view of a steel cord;

FIG. 6 is a perspective view schematically illustrates a sample used in a pull-out test;

FIG. 7 illustrates chromatograms of respective ones of a Comparison Example 1, Comparison Example 2 and an Example 5;

FIG. 8 illustrates, using an enlarged retention time scale, chromatograms of respective ones of a Comparison Example 1, Comparison Example 2 and Example 5; and

FIG. 9 is a graph illustrating the relationship between peak intensity ratio and pull-out load.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 1, FIG. 1 is a block diagram schematically illustrating a process for surface treatment of a brass-plated steel cord 1A. FIGS. 2 to 4 schematically illustrate, by means of structural formulae, chemical bonding that occurs on the surface of the steel cord 1A by subjecting the steel cord to the surface treatment process shown in FIG. 1. FIG. 5 is an enlarged cross-sectional view of the steel cord 1A. Illustration of hatching indicating a cross-section is omitted in FIG. 5.

With reference to FIG. 5, the steel cord 1A is obtained by twisting together seven brass-electroplated high-carbon steel strands (steel wires) 3, thereby forming a twisted wire 2, and furthermore twisting together seven of the twisted wires 2. The wires 3 used have a diameter of 0.35 mm, by way of example. The steel cord 1A having this construction is denoted “7×7×0.35HT” (where HT is the abbreviation of “High Tensile”). The structure and diameter of the steel cord 1A can be designed as appropriate.

The long steel cord 1A has been wound around a delivery bobbin (not shown). The steel cord 1A is delivered from the delivery bobbin at a constant speed.

With reference to FIG. 1, the steel cord 1A delivered from the delivery bobbin is initially passed through an organic solvent tank 11 filled with an organic solvent 13. Owing to immersion in the organic solvent 13, oil (lubricant), especially non-water soluble oil, which adheres to the steel cord 1A when the steel cord 1A is processed (at the time of manufacture or at the time of molding), dissolves in the organic solvent 13 and is removed. The surface treatment in which the steel cord 1A is immersed in the organic solvent 13 is referred to as “degreasing treatment” below. The steel cord 1A is manufactured by twisting together a plurality of the twisted wires 2 each of which is obtained by twisting together the plurality of wires 3 in the manner described above. The wires 3 constituting the steel cord 1A generally are slimmed down to a predetermined diameter by being subjected to a wire drawing process, and the slimmed-down wires 3 are twisted together. During the processing of the steel cord 1A that includes the wire drawing process and the twisting process, the wires 3 and twisted wires 3 are coated with lubricants of different types, and there are instances where use is made of both water-soluble and non-water-soluble lubricants as the lubricants applied. The organic solvent 13 mainly removes non-water-soluble oil that has adhered to the steel cord 1A in the process of manufacturing the steel cord 1A. Depending upon the components of the non-water-soluble oil adhering to the steel cord 1A, the organic solvent 13 used is hydrocarbon-based, ketone-based, alcohol-based, ester-based or ether-based.

The organic solvent tank 11 is placed inside a water tank 12 (referred to below as ultrasonic cleaning tank 12) filled with water 14 and equipped with an ultrasonic generator (not shown). Ultrasonic waves generated by the ultrasonic cleaning tank 12 propagate toward the organic solvent 13 through the water 14. By immersing the steel cord 1A in the ultrasonically vibrated organic solvent 13, non-water-soluble oil that has entered into the gaps between the twisted wires 2 constituting the steel cord 1A (the helical recess in the surface of the steel cord 1A) and into the gaps between the wires 3 can be removed efficiently in a short period of time. It should be noted that the reason for not storing the ultrasonic cleaning tank 12 directly in the organic solvent 13 is to prevent the ignition of the organic solvent 13 in which heat is produced by the ultrasonic vibration. Typically, the steel cord 1A is immersed in the organic solvent 13 on the order of 30 seconds. The immersion time can be suitably adjusted depending on the feeding speed of the steel cord 1A and on an increase or decrease in the number of times the steel cord 1A is repeatedly engaged alternately with a pair of rollers provided in the organic solvent tank 11.

The steel cord 1A cleaned using the organic solvent 13 is passed through an aqueous alkaline solution tank 21 filled with an aqueous alkaline solution 22.

By immersing the steel cord 1A in the aqueous alkaline solution 22, water-soluble oil adhering to the steel cord 1A is saponified and removed. Further, since the steel cord 1A is brass-plated, as described above, copper (or copper oxide) and zinc (or zinc oxide) contained in the brass plating are present on the surface of the cord. When the brass plating comes into contact with the aqueous alkaline solution 22, the zinc in the brass plating and a base in the aqueous alkaline solution chemically react and a hydroxyl group (—OH) is introduced onto the surface of the steel cord 1A (see FIG. 2). The surface treatment in which the steel cord 1A is immersed in the aqueous alkaline solution 22 will be referred to as a “hydroxide treatment” below.

When the alkali concentration of the aqueous alkaline solution 22 is too low, the reaction rate between the zinc and base slows down and the introduction of the hydroxyl group may be insufficient. The alkali concentration sought, therefore, is pH 11 or higher, preferably pH12 or higher. Conversely, when the alkali concentration is too high, a non-uniformity occurs in the chemical reaction, the copper in the brass plating becomes discolored and cleaning takes time. An alkali concentration of less than pH 13.5, therefore, is suitable. In a case where aqueous sodium hydroxide is adopted as the aqueous alkaline solution 22, the concentration of the aqueous sodium hydroxide is made 0.002 mass % or more and less than 1 mass % for example, preferably 0.02 mass % or more and less than 1 mass %.

Typically, the steel cord 1A is immersed in the aqueous alkaline solution 22 on the order of from 30 seconds to 1 minute. The temperature of the aqueous alkaline solution 22 is set as appropriate and use at room temperature (about 25° C.) poses no problem.

Ultrasonic vibration may be applied to the aqueous alkaline solution tank 21 as well and the steel cord 1A may be immersed in the ultrasonically vibrated aqueous alkaline solution 22.

With reference to FIG. 2, oil (especially water-soluble oil) 61 (shown having the chemical structure of oleic acid in FIG. 2) is not removed completely from the surface of the steel cord 1A (brass plating) and a small amount remains. In order to remove the oil 61 from the surface of the steel cord 1A completely or substantially completely, it is necessary to immerse the steel cord 1A in the aqueous alkaline solution 22 for a long period of time. If the cord is immersed for a long period of time, however, production yield declines and, in addition, there are instances where the brass plating becomes discolored and gloss is lost as well. To prevent this, it will suffice to avoid long immersion of the steel cord 1A in the aqueous alkaline solution 22. It should be noted that no particular problems arise in performance even if some oil 61 remains on the surface of the steel cord 1A. This will be explained in detail in test examples described below.

The steel cord 1A that has been passed through the aqueous alkaline solution tank 21 proceeds to an aqueous silane coupling solution tank 31 after being passed through and washed, as necessary, in a water-washing tank (not shown) filled with water.

The aqueous silane coupling solution tank 31 is filled with an aqueous silane coupling solution 32 obtained by dissolving in water an aqueous silane coupling agent, for example an amino-based silane coupling agent that includes an amino group (—NH₂) as a functional group. The silane coupling agent includes a hydrolyzable group [an alkoxyl group (RO—), for example] and, by being dissolved in water, is hydrolyzed into a silanol group (Si—OH). When the steel cord 1A having the hydroxyl group introduced onto its surface is immersed in the aqueous silane coupling solution 32, the silanol group forms hydrogen bonds with the hydroxyl group on the surface of the steel cord 1A and the silane coupling agent adheres to the surface of the steel cord 1A (a film of the silane coupling agent forms on the steel cord 1A) (see FIG. 3). The surface treatment in which the steel cord 1A is immersed in the aqueous silane coupling solution 32 will be referred to a “silane coupling agent treatment” below.

Although the concentration of the silane coupling agent in the aqueous silane coupling solution 32 is not particularly limited, 1 to 8 volume %, for example, is suitable and 2 to 4 volume % even more suitable. The reason for this is that, with less than 1 volume %, lengthening of immersion time and temperature control are required, and when 8 volume % is exceeded, there are instances where a large amount of silane coupling agent adheres and film thickness becomes excessive. Although the temperature of the aqueous silane coupling solution 32 is not particularly limited, 20 to 40° C. is suitable and no problems arise at room temperature (25° C.). Immersion time is not particularly limited but 30 seconds or more is suitable and one minute or longer more suitable. Making the immersion time longer than necessary is not required, though.

It should be noted that since the silanol groups included in the aqueous silane coupling solution 32 precipitate in the aqueous solution with the passage of time, it is preferred that the aqueous silane coupling solution 32 be stirred suitably by a stirrer (not shown).

The steel cord 1A that exits from the aqueous silane coupling solution tank 31 is passed through a heating oven 41. SiO bonds are formed on the surface of the steel cord 1A (brass plating) by a dehydration condensation reaction between silanol groups derived from the silane coupling agent and hydroxyl groups on the surface of the steel cord 1A, whereby the silane coupling agent is chemically bonded (fixed) strongly to the surface of the steel cord 1A (see FIG. 4). The heating temperature in the heating oven 41 is made 110° C., by way of example, and the heating time is made a time suitable for the dehydration condensation reaction, say 5 minutes by way of example.

The steel cord 1A having the silane coupling agent chemically bonded to its surface is taken up by a winding bobbin (not shown). The steel cord 1A in its final state to be taken up by the winding bobbin will be referred to below as “steel cord 1B with the silane coupling agent” or as “surface-treated steel cord 1B”.

Table 1 illustrates the results of pull-out tests on 10 types of specimen (steel cord) manufactured by changing whether or not the above-described surface treatments (degreasing treatment, hydroxide treatment, silane coupling agent treatment and heat treatment) are applied to the steel cord as well as the method of surface treatment and degree (length thereof). In degreasing treatment and hydroxide treatment, it should be noted, treatment in which mere immersion is performed (IMMERSION) and in which the sample is immersed while being ultrasonically vibrated (IMMERSION+ULTRASOUND) are distinguished from each other. The steel cord used was the 7×7×0.35HT steel cord mentioned above. Further, Table 1 summarizes test results regarding specimens (referred to as “COMPARISON EXAMPLES”) not subjected to degreasing treatment, and test results regarding specimens (referred to as “EXAMPLES”) subjected to degreasing treatment.

Table 1 furthermore illustrates, with regard to each of the specimens, peak intensity (a) of an amino compound (propyl amine as one example), peak intensity (b) of an oil (water-soluble oil) (oleic acid as one example), and a value (a/b) (referred to as “PEAK INTENSITY RATIO A”), which is obtained by dividing the peak intensity (a) of the amino compound by the peak intensity (b) of the oil, these being obtained as the result of separation analysis by pyrolysis gas chromatography. It should be noted that peak intensity corresponds to the amount of ions produced by ionization of the object under analysis (the amount relative to all ions).

TABLE 1 HYDROXIDE SILANE COUPLING DEGREASING TREATMENT AGENT TREATMENT TREATMENT (NaOH 1 MASS %) (2 VOL. %) CLEANING TIME CLEANING TIME IMMERSION METHOD (SEC) METHOD (SEC) TIME (SEC) COMPARISON NONE NONE NONE EXAMPLE 1 COMPARISON NONE IMMERSION 30 30 EXAMPLE 2 COMPARISON NONE IMMERSION + 10 30 EXAMPLE 3 ULTRASOUND COMPARISON NONE IMMERSION + 30 30 EXAMPLE 4 ULTRASOUND EXAMPLE 1 IMMERSION 10 IMMERSION 30 30 EXAMPLE 2 IMMERSION 30 IMMERSION 30 30 EXAMPLE 3 IMMERSION + 5 IMMERSION 30 30 ULTRASOUND EXAMPLE 4 IMMERSION + 10 IMMERSION 30 30 ULTRASOUND EXAMPLE 5 IMMERSION + 30 IMMERSION 30 30 ULTRASOUND EXAMPLE 6 IMMERSION + 30 IMMERSION + 30 30 ULTRASOUND ULTRASOUND HEAT TREATMENT PEAK INTENSITY PEAK TEMPARATURE/ AMINO INTENSITY PULL-OUT TIME COMPOUND (a) OIL (b) RATIO (a/b) LOAD (N) COMPARISON NONE — 2,359,566 0.0 386.0 EXAMPLE 1 COMPARISON 110° C./5 MIN 1,106,047 931,408 1.2 651.3 EXAMPLE 2 COMPARISON 110° C./5 MIN 1,050,819 815,853 1.3 676.9 EXAMPLE 3 COMPARISON 110° C./5 MIN 1,321,430 638,372 2.1 725.9 EXAMPLE 4 EXAMPLE 1 110° C./5 MIN 1,476,738 573,268 2.6 764.2 EXAMPLE 2 110° C./5 MIN 1,522,785 542,689 2.8 775.4 EXAMPLE 3 11C° C./5 MIN 1,443,194 603,342 2.4 745.0 EXAMPLE 4 110° C./5 MIN 1,627,316 360,984 4.5 948.5 EXAMPLE 5 110° C./5 MIN 1,696,080 372,563 4.6 950.5 EXAMPLE 6 110° C./5 MIN 1,688,444 370,273 4.6 951.2

More specifically, Finesolve E manufactured by Sankyo Chemical Co. was employed as the organic solvent 13 used in the degreasing treatment. Aqueous sodium hydroxide having a concentration of 1 mass % was employed as the aqueous alkaline solution 22 used in the hydroxide treatment. An aqueous amino organosilane solution having a concentration of 2 volume % was employed as the aqueous silane coupling solution 32 used in the silane coupling agent treatment. A multishot pyrolizer (stock number: EGA/PY-3030D) manufactured by Frontier Laboratories Ltd. was used in the pyrolysis gas chromatography.

FIG. 6 illustrates the external appearance of a sample used in a pull-out test. With regard to each of the 10 types of steel cord in the pull-out test, one end portion thereof was covered with urethane resin 51 in the form of a cylinder having a height of 12.7 mm and a diameter of 8 mm, thereby fabricating a sample in which the steel cord is embedded in the urethane resin 51. Three samples were fabricated with regard to each of the 10 types of steel cord and the load necessary for pull-out of the steel cord was measured for every sample using a pull-out tester. The average value of the pull-out loads measured for respective ones of the three samples is indicated as PULL-OUT LOAD in Table 1.

When the urethane resin 51 adheres to the steel cord 1B with the silane coupling agent, an ureido reaction occurs and the silane coupling agent and urethane resin 51 chemically bond strongly. Whereas the pull-out load regarding the sample (Comparison Example 1) in which the steel cord 1A not subjected to any surface treatment is embedded in the urethane resin 51 is 386.0N, this value is exceeded greatly by all of the pull-out loads regarding the samples (Comparison Examples 2 to 4 and Examples 1 to 6) in which the steel cord 1B with the silane coupling agent subjected to at least the hydroxide treatment and the silane coupling agent treatment are embedded in the urethane resin 51. It will be understood that the steel cord 1B with the silane coupling agent and the urethane resin 51 are strongly bonded.

When only the hydroxide treatment, silane coupling agent treatment and heat treatment are performed without applying the degreasing treatment in which the steel cord 1A is immersed in the organic solvent 13 (Comparison Examples 1 to 4), the pull-out load is 725.9N at most (Comparison Example 4). In contrast, this value is exceeded by all of the pull-out loads (Examples 1 to 6) when, in addition, the degreasing treatment in which the steel cord 1A is immersed in the organic solvent 13 is applied. It will be understood that the steel cord 1B with the silane coupling agent and the urethane resin 51 can be adhered together more strongly by performing the degreasing treatment.

By comparing Examples 1 and 2 and by comparing Embodiments 3 to 5 with each other, it is seen that a difference in pull-out load arises due to the length of time the steel cord 1A is immersed in the organic solvent 13. It is considered that when the period of time over which the steel cord 1A is immersed in the organic solvent 13 is too short, much oil remains on the steel cord 1A and this affects the pull-out load. With reference to Examples 3 to 5, in comparison with the difference (948.5−745.0=203.5N) between the pull-out load (745.0N) (Example 3) when the immersion time in the organic solvent 13 is 5 seconds and the pull-out load (948.5N) (Example 4) when the immersion time is 10 seconds, the difference (950.5−948.5=2.000N) between the pull-out load (748.5N) (Example 4) when the immersion time is 10 seconds and the pull-out load (950.5N) (Example 4) when the immersion time is 30 seconds is fairly small. It will be understood that the immersion time in the organic solvent 13 need not be made longer than necessary and that 30 seconds is sufficient.

Furthermore, contrasting Example 2 and Example 5, it will be understood that additionally applying ultrasonic vibration to the organic solvent 13 affects the final pull-out load and is associated with the strength of adhesion between the steel cord 1B with the silane coupling agent and the urethane resin 51. In order to improve adhesion strength, it can be said that immersing the steel cord 1A in the ultrasonically vibrated organic solvent 13 for a predetermined period of time (on the order of 30 seconds) is effective.

Furthermore, contrasting Example 5 and Example 6, it is seen that, when the steel cord is immersed in the ultrasonically vibrated organic solvent 13 over a predetermined period of time, no significant difference in pull-out load occurs regardless of whether the aqueous alkaline solution 22 in which the steel cord is immersed next is not ultrasonically vibrated (Example 5) or is ultrasonically vibrated (Example 6). Applying the ultrasonic vibration does slightly improve the pull-out strength, though.

FIGS. 7 and 8 illustrate results (chromatograms) of a separation analysis test of surface substances with regard to respective ones of three types of specimen among the ten types of specimen, particularly the specimens of Comparison Example 1 (upper), Comparison Example 2 (middle) and Example 5 (lower). The vertical and horizontal axes in the chromatograms indicate substance signal strength and retention time, respectively. Since retention time from the moment in time the sample is introduced to pyrolysis gas chromatography to the moment in time at which the separated substance indicates a peak is a specific value that depends on the substance, the substance can be identified by the retention time indicating the peak and the peak intensity (relative amount of ions) of this peak and be measured. It should be noted that the scale of retention time in FIG. 7 differs from that of FIG. 8.

In the chromatograms, the oil (especially water-soluble oil) (oleic acid as one example) peaks at a retention time of about 25 minutes (FIG. 7), and the amino compound (propyl amine as one example) peaks at retention times of about 1.60 minutes and 1.75 minutes (FIG. 8). Based on these peaks that appear in the chromatograms, the peak intensity a of the amino compound and the peak intensity b of the oil can be measured (calculated) for each specimen. (The peak intensity a was calculated using the sum of two peaks.) The specific chemical component of the oil for which a peak appears is determined in accordance with the lubricant used at the time of processing of the steel cord 1A. The specific chemical component of the amino compound for which a peak appears is determined in accordance with the amino-based silane coupling agent used in the aqueous silane coupling solution 32.

FIG. 9 shows a graph in which, using the peak intensity a of the amino compound and the peak intensity b of the oil measured by chromatography with regard to each specimen, peak intensity ratio A, which is obtained by dividing the peak intensity a of the amino compound by the peak intensity b of the oil, is plotted along the horizontal axis, and pull-out load (in N units), which is measured by the pull-out test regarding each specimen, is plotted along the vertical axis. The plurality of circles illustrated in the graph indicate the relationship between peak intensity ratio and pull-out load with regard to each specimen.

With reference to the graph of FIG. 9, it will be understood that the pull-out load and the peak intensity ratio A are correlated, and the larger the peak intensity ratio A, the larger the pull-out load. If the peak intensity ratio A is 2.6 are more, it is possible to achieve a pull-out load that is about twice the pull-out load obtained when use is made of a steel cord subjected to no surface treatment at all.

In the embodiment described above, an amino silane coupling agent that includes an amino group is exemplified from the standpoint of water-solubility, and the urethane resin 51 is exemplified as an organic material made to adhere to a steel cord via the amino silane coupling agent. However, a resin other than the urethane resin 51, for example, polystyrene, acryl, polyvinyl chloride, nylon, phenol, epoxy, furan and other resins will bond strongly with the amino silane coupling agent. By using the steel cord 1B with the silane coupling agent that has undergone the above-described surface treatment, firm adhesion can be achieved also with a product formed from these resins.

Further, in the embodiment described above, the surface-treated steel cord 1B is fabricated by applying surface treatments that include a degreasing treatment, hydroxide treatment, silane coupling agent treatment and heat treatment to the steel cord 1A obtained by twisting together seven of the twisted wires 2 that are the result of twisting seven of the wires 3 together. However, the surface-treated steel cord 1B can also be fabricated by applying the surface treatment to the wire 3 or twisted wire 2 and twisting these together. 

What is claimed is:
 1. A method of surface-treating a zinc-containing metal substrate, comprising: bringing an organic solvent into contact with the surface of a substrate having a zinc-containing metal on a surface layer thereof; bringing an aqueous alkaline solution into contact with the surface of the substrate that was brought into contact with the organic solvent; bringing an aqueous silane coupling agent solution into contact with the surface of the substrate that was brought into contact with the aqueous alkaline solution; and heating the substrate brought into contact with the aqueous silane coupling agent solution.
 2. A method of surface-treating a zinc-containing metal substrate according to claim 1, wherein: an organic solvent tank filled with the organic solvent is placed inside a water tank filled with water; said water tank is subjected to ultrasonic vibration; and said substrate is immersed in the organic solvent vibrated ultrasonically via the water.
 3. A method of surface-treating a zinc-containing metal substrate according to claim 1, wherein: an aqueous alkaline solution tank filled with the aqueous alkaline solution is subjected to ultrasonic vibration; and said substrate is immersed in the aqueous alkaline solution vibrated ultrasonically.
 4. A method of surface-treating a zinc-containing metal substrate according to claim 1, wherein said aqueous alkaline solution is aqueous sodium hydroxide.
 5. A method of surface-treating a zinc-containing metal substrate according to claim 1, wherein said silane coupling agent is an amino-based silane coupling agent.
 6. A method of surface-treating a zinc-containing metal substrate according to claim 1, wherein said substrate is a filament having a surface plated with a zinc-containing metal.
 7. A method of surface-treating a zinc-containing metal substrate according to claim 1, wherein said substrate is a twisted wire obtained by twisting together multiple filaments each of which has a surface plated with a zinc-containing metal.
 8. A surface-treated zinc-containing metal substrate wherein an amino compound and oil are adhered to the surface of a substrate that has a zinc-containing metal on a surface layer thereof, characterized in that: a peak intensity ratio A represented by A=a/b is equal to or greater than 2.6, where a is peak intensity of the amino compound and b is peak intensity of the oil.
 9. A surface-treated zinc-containing metal substrate according to claim 8, wherein the surface of said substrate is plated with a zinc-containing metal.
 10. A surface-treated zinc-containing metal substrate according to claim 8, wherein said substrate is a twisted wire obtained by twisting together multiple filaments each of which has a surface plated with a zinc-containing metal. 